Case C Coffee Percolator

A coffee percolator operates by circulating a stream of boiling coffee solution from the reservoir in the base of the coffee pot up through a central rise-pipe to the top of a bed of coffee granules, through which the solution then percolates. 

The concentrations of the coffee, both in the granules and in the liquid flowing through the bed, will vary continuously both with distance and with time. The behaviour of the packed bed is therefore best approximated by a series of many uniform property subsystems. Each segment of solid is related to its appropriate segment of liquid by interfacial mass transfer, as shown in Fig. 1.9. 

The resulting model would therefore consist of component balance equations for the soluble component written over each of the many solid and liquid subsystems of the packed bed, combined with the component balance equation for the coffee reservoir. The magnitude of the recirculating liquid flow will depend on the relative values of the pressure driving force generated by the boiling liquid and the fluid flow characteristics of the system. 

The concept of modelling a coffee percolator as a dynamic process comes from a problem suggested by Smith et al. (1970) and extended by Ramsay (Bradford University). 

Finite difference elements for the solid coffee grounds 

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Maier (1973) studied the physical sorption of volatile aroma constituents by foods using IR spectroscopy. He observed that aliphatic aldehydes such as propanal (C.3), hexanal (C.6), acrolein (C.16) were also reacting chemically with some amino compounds, specially with cysteine (formation of thiazolidine carboxylic acids), with other amino acids, with glutathione and with urea. The bound aldehydes are nevertheless, in most cases, released by heating with water, especially during the percolation of the coffee beverage. 

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